Requirements:
-Jtag/RGH/Dev-kit
-Computer
-Assembly (make sure its set to Wumbo) Part 1: Preparation

Step 1: Open Assembly and goto Tools>Settings>Map Editor
under Game Specific Resource Cache change selected game to Halo 3


Step 2: Find the directory that has you clean maps or has a clean campaign.map and shared.map
EX:E:\Halo\Halo3\Maps\
I'm not 100% positive if you have to have the \ on the end to tell it its a directory or not.

Step 3: Now open the map that you want to use to get your bipd I'm going to use 100_citadel.map
I'm going to be using the hunter for this entire tutorial just apply it the same way for other bipds

Step 4: Goto Tags>bipd then select objects\characters\hunter\hunter now right click and select Extract... (With Raw)
Save and name the tag Where you want
I'm going to use a made up directory such as saving it in E:\Halo\Halo3\Tags\

Part 2: Injection and Adding

Step 1: Open up the map that you want to inject the bipd into. I'm going to be using shrine.map for my map.

Step 2: Goto tags then next to Save Tag Names click Import... then select the tag you saved in Part 1 Step 4

Step 3: Now we are going to add a custom marker so that we are replacing or using a current one so that also it can familiarize you guys with adding custom string and using them
I'm going to be using then English strings so goto Strings>English​ and then click on the drop down box next to String List. I'm going to be adding my string to ui\global_strings\global_strings now click Add New at the bottom I'm going to name my string Custom0015


  Optional: if you want it to say hold <_button_action_reload> to Drive bipd or what ever you want
then open the string list ui\hud\hud_messages and add a custom string here also. adding it here
make it easy to keep things clean.
Step 4: Now goto tags>bipd>objects\characters\hunter\hunter (or your custom bipd)

Step 5: Scroll down to Seats next to count click the + in between the count and the I
now next to New Entry count: change to 1 or you can use Quick Add...
Hunter will already have a seat
​the seat should look like this make sure you uncheck boarding seat
also Invisible is optional all id does is when your fully in the vehicle it makes you invisible  

Step 6: Now scroll down and expand the Camera Track     Part 3: Fixing Things Up
Step 1: Now that we have the seats added change the Seat animation under Seats to something like warthog_d

Step 2: Change the seats marker to Custom0015 and change Enter Marker Name to Custom0015
I you want to know what these are a bit more
-Seats animation is the animation that plays when you enter a vehicle
EX: change the warthog to a different vehicles seat animation and you will see a different animation
-Seat marker is where your character sits when the character is in the vehicle
-Seat entry marker is where you enter the vehicle
EX: when it says Hold RB to Drive Warthog

Optional Continued: If you added the custom enter string. then scroll down to Enter Seat String
and enter the custom sting you added to in ui\hud\hud_messages if you chose to add it into that String List
Note: You may have already did this in Part 2 Step 5 if you followed the picture.

Step 3: Now open the Hunter (or your bipds) mode tag and scroll down to marker groups click the + next to the count and add 1
this should pop up.


  Step 4: Go down to the new added count in my case I'm going to be going to 18-18 (19) since that is our newly added count.



Step 5: Now we are going to add a count of 1 to the markers make sure you have the newest marker group you added selected markers
should be grayed out and the count box should have no number in it currently now click the + next to the i
and make the new count 1 or you can do Quick Add...



Step 6: Now have the new marker group selected and change the Name to Custom0015 (or your custom String you added in Part 2 Step 3)




Now when you spawn your bipd you should now be able to walk up to the bipd hold the action button and enter the bipd

if there are any errors or anything I left out please let me know I may have left something out by accident not even thinking about it. but i am pretty sure everything is here.   As always thanks to the whole Assembly team for Assembly.